Method for Making Ultra-Fine Carbon Fibers and Activated Ultra-Fine Carbon Fibers

ABSTRACT

A method for making ultra-fine carbon fibers and activated ultra-fine carbon fibers includes core-sheath melt-spinning to produce a core-sheath fiber including a sheath made of polyolefin and a carbon-containing polymer, and a core made of a polyolefin polymer. Under control of temperature, the core-sheath fiber is stabilized as a carbon-containing precursor fiber. The stabilized core-sheath fiber is carbonized in nitrogen at 600 to 1500 degrees Celsius and converted into ultra-fine carbon fibers with a diameter of 20 to 800 nm. Then, the ultra-fine carbon fibers are activated in carbon dioxide, steam, air or any combination thereof at 600 to 1500 degrees Celsius to form activated ultra-fine carbon fibers.

BACKGROUND OF THE INVENTION

The present invention relates to a method for making ultra-fine carbon fibers and activated ultra-fine carbon fibers.

Carbon fibers are commonly used in the aerospace industry, the recreational equipment industry and many other industries because of excellent mechanical properties such as specific strength and specific elastic module. The performance of the carbon fibers may not be good enough for applications that require excellent strength, fineness, light weight and thermal and electrical conductivities. There is an increasing need for ultra-fine carbon fibers and activated ultra-fine carbon fibers for use in reinforced composite patches, containers for hydrogen, lithium cell electrodes, ultra-high capacitors and filters.

To improve the mechanical properties, techniques have been devised to blend various compounds, in the form of fine particles, in carbon fibers. Examples can be found in Japanese Patent Publication No. 1986-58404 and Japanese Patent Application Publication Nos. 1990-251615 and 1992-272236. The fine particles blended in single fiber are impurities and, thus, entail the breach during the making of the carbon fibers in the procedure of production of single fiber or calcination of carbon fibers, thus reducing the throughput and deteriorate the mechanical properties such as the tensile strength. By reducing the size of the graphite crystals for making the carbon fibers, the strength against compression can be improved and the axial strength between the graphite crystals may also be increased. If fine particles of metal are blended in the carbon fibers, the strength against compression will however deteriorate due to growth of the crystals resulting from graphitization of catalyst.

Alternatively, techniques have been devised to mix various sorts of resin with polyacrylic polymers. Examples can be found in Japanese Patent Application Publication No. 1993-195324 and Taiwanese Patent Publication No. 561207. It is however difficult to achieve evenly configured carbon fiber, and the strength is often low.

Japanese Patent Application Publication No. 1991-180514 discloses a technique for ionizing gaseous atoms or molecules, accelerating the ions in an electric field and injecting the accelerated ions into the superficial portions of the carbon fibers for improving the mechanical properties. It however requires vacuum and fails to make ultra fine carbon fibers and is therefore not ready for the industry.

Taiwanese Patent Publication No. 73021 discloses vapor phase growth of nanometer-scale carbon fibers. At a high temperature, carbon-containing gas is thermally decomposed on metal catalyst and made into nanometer-scale carbon fibers. Thus, nanometer-scale ultra-fine carbon fibers are made of inexpensive carbon-containing gas in a one-step process. This method can be referred to as chemical vapor deposition (“CVD”). The throughput is however low. Improvement is therefore needed.

A method for making ultra-fine carbon fibers was disclosed by Asao Oya in Japanese Functional Material April 2000, Vol. 20, No. 4, PP 20-26. The fine particles are made by mixing carbon-containing polymer with heat-decomposed polymer in solvent. To form the fine particles is to dissolve the carbon-containing polymer in solvent and then to spray the carbon-containing polymer solution into the solution of another heat-decomposed polymer so that the micrometer-scale particles are formed onto the surface of carbon-containing polymer wrapped by heat-decomposed polymer. This method is however complicated and entails environment pollution since the raw materials must be dissolved in the solvent. The carbon-containing precursor of ultra-fine carbon fibers was disclosed by Asao Oya through mixing carbon-containing polymer (phenolic) and heat-decomposed polymer (polyethylene) in solvent to form the fine particles, and then blend with heat-decomposed polymer through the procedure of melting, stabilizing and carbonization, the ultra-fine carbon fibers are made.

Micrometer-scale particles are made by dissolving phenolic in solvent and wrapped by polyethylene. The solvent is removed to obtain phenolic-polyethylene particles. The phenolic-polyethylene particles are mixed with polyethylene in the ratio of 3:7, melted at 150 degrees Celsius and spun to form fibers. The fibers are stabilized in an acid environment, neutralized in ammonia water, washed in de-ionized water and dried. Thus, the diameter of the fibers is tens of micrometers. The fibers are carbonized at 600 degrees Celsius for 10 minutes so that the polyethylene is decomposed and that ultra-fine carbon fibers are derived from the phenolic resin. The diameter of the ultra-fine carbon fibers is 200 to 250 nanometers. This method is however complicated and entails environment pollution since the phenolic must be dissolved in the solvent and wrapped by the polyethylene and the solvent must be removed.

A method for making ultra-fine carbon fibers from phenolic is disclosed in Japanese Patent Publication No. 2001-73226. Phenolic resin is mixed with polyethylene and melted at 120 to 160 degrees Celsius so that plastic particles are made. Fibers are made of the plastic particles by melt-spinning at 120 to 200 degrees Celsius. The fibers are stabilized in an acid environment at 96 degrees Celsius for 24 hours, neutralized by ammonia water, washed in de-ionized water and dried so that the fibers are made with a diameter of tens of micrometers. The fibers are carbonized in nitrogen at 600 degrees Celsius for 10 minutes so that the polyethylene is decomposed and that ultra-fine carbon fibers are derived from the phenolic resin. It however takes a lot of time to mix the phenolic resin with the polyethylene and melt the mixture. For example, it takes about 50 minutes to make and melt 100 grams of the mixture of the phenolic resin with the polyethylene.

The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide an efficient method for making ultra-fine carbon fibers and activated ultra-fine carbon fibers.

It is another objective of the present invention to provide a simple method for making ultra-fine carbon fibers and activated ultra-fine carbon fibers.

It is still another objective of the present invention to provide an in-expensive method for making ultra-fine carbon fibers and activated ultra-fine carbon fibers.

According to the present invention, a method is provided for making ultra-fine carbon fibers and activated ultra-fine carbon fibers. By core-sheath melt-spinning, there is made a core-sheath fiber including a sheath made of polyolefin and a carbon-containing polymer, and a core made of a polyolefin polymer. Under control of temperature, the core-sheath fiber is stabilized as a carbon-containing precursor fiber. The carbon-containing precursor fiber is carbonized in nitrogen at 600 to 1500 degrees Celsius, and converted into ultra-fine carbon fibers with a diameter of 20 to 800 nm. Then, the ultra-fine carbon fibers are activated in carbon dioxide, steam, air or any combination of them at 600 to 1500 degrees Celsius, and made into activated ultra-fine carbon fibers.

Other objectives, advantages and features of the present invention will become apparent from the following description referring to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described via detailed illustration of the preferred embodiment referring to the drawings.

FIG. 1 is a cross-sectional view of a core-sheath fiber used in a method for making ultra-fine carbon fibers according to the preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view of another core-sheath fiber used in a method for making ultra-fine carbon fibers according to the preferred embodiment of the present invention.

FIG. 3 is a cross-sectional view of still another core-sheath fiber used in a method for making ultra-fine carbon fibers according to the preferred embodiment of the present invention.

FIG. 4 is a cross-sectional view of yet another core-sheath fiber used in a method for making ultra-fine carbon fibers according to the preferred embodiment of the present invention.

FIG. 5 is a cross-sectional view of the core-sheath fiber used in this method for making ultra-fine carbon fibers according to the preferred embodiment of the present invention. (21: carbon-containing polymer, 22: polyolefin, 23: polyolefin)

FIG. 6 is a Scanning electron-microscopic photograph of ultra-fine carbon fibers according to the present invention.

FIG. 7 is a Transmission electron-microscopic photograph of single ultra-fine carbon fiber according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a method is provided for making ultra-fine carbon fibers and activated ultra-fine carbon fibers. By core-sheath melt-spinning, there is made a core-sheath fiber including a sheath made of polyolefin and a carbon-containing polymer, and a core made of a polyolefin polymer. Under control of temperature, the core-sheath fiber is stabilized as a carbon-containing precursor fiber. The carbon-containing precursor fiber may be carbonized in nitrogen at 600 to 1500 degrees Celsius, and converted into ultra-fine carbon fibers with a diameter of 20 to 800 nm.

Alternatively, the ultra-fine carbon fibers may be activated in carbon dioxide, steam, air or any combination thereof at 600 to 1500 degrees Celsius, and made into activated ultra-fine carbon fiber.

Alternatively, the ultra-fine carbon fibers may be graphitized in argon at 1500 to 3000 degrees Celsius, and made into ultra-fine graphite fibers.

Referring to FIGS. 1 through 4, there are shown various cross-sections of core-sheath fiber used in the method according to the present invention. Each of these core-sheath fibers includes at least two polymers, and is melted and spun through a core-sheath nozzle. The weight ratio of the sheath to the core ranges from 90:10 to 10:90. The sheath and the core may be co-centric as shown in FIGS. 1 and 2 or eccentric as shown in FIG. 3 or tri-lope as shown in FIG. 4.

The carbon-containing polymers include phenol-formaldehyde, thermoplastic polyacrylonitrile, thermoplastic polyvinyl alcohol, polyvinyl chloride and mesophase pitch for example.

The polyolefin may be polystyrene, polyethylene, polypropylene, polymethylpentene or an olefin-containing co-polymer.

By spinning, the phenolic fibers are made of phenolic resin that includes a molecular weight of 500 to 2000. Generally, uncured phenolic fibers can be made by wet spinning or dry spinning. However, the uncured phenolic fibers with low molecular weight cannot easily be made by melt-spinning because the phenolic fibers can easily be broken. Uncured phenolic fibers may be cured in acid catalyst such as hydrochloric acid and phosphoric acid or aldehyde such as formaline, polyformaline, trioxane and tetraoxane. Alternatively, the superficial portions of the phenolic fibers that have not been cured in the foregoing solution may be pre-cured and then neutralized in alkaline solution such ammonia water, hexamethylene tetraammine solution, urea solution and potassium hydroxide solution to have the phenolic fibers cured. Generally, the acid catalyst, alkaline solution, aldehyde are hydrochloric acid, ammonia water and formaline, respectively. The cured phenolic fibers are suitable for use as precursor for carbon fibers because they cannot be melted and include a lot of carbon.

The phenolic resin of the carbon-containing polymer used in the method according to the present invention is made with a molecular weight of 2000 to 10000. Thus, the phenolic resin and the polyolefin, which can easily be melt-spinning, can be made into un-stabilized core-sheath fibers by core-sheath nozzles.

If gasoline pitch, carbon pitch, isotropic pitch or anisotropic pitch is melted and spun alone, it will be difficult to make carbon-containing precursor in the form of fibers smaller than 5 micrometers in diameter due to resistance from air. This is because the viscosity of the pitch is highly dependent on the temperature. Pitch therefore cannot be extended to a fineness of 5 to 15 micrometers as easily as ordinary high-molecular compound. The throughput is low. The back pressure of the nozzle is lower for pitch than for ordinary high-molecular compound so that the nozzle can easily be jammed if there are highly viscous impurities. To avoid this, highly viscous impurities must be removed from gasoline heavy oil or carbon heavy oil. Even though impurity solids have been removed from the pitch, the viscosity of the pitch may still become high because of heating and oxidization such as distilling and mesophase bituminizing and jam the nozzle. The drawback may therefore be overcome by using mesophase pitch as material for spinning.

To make mesophase pitch for use in the method according to the present invention, with nickel-molybdenum catalyst used, coal tar is brought into contact with hydrogen at 400 degrees Celsius for 120 minutes. The resultant hydrogenised coal tar is filtered by a 1 micrometer filter so that solids are removed. Then, the hydrogenised coal tar is distilled at 350 degrees Celsius so that hydrogenised pitch is made. The hydrogenised pitch is processed at 520 degrees Celsius and 17 mmHg for 7 minutes so that the mesophase pitch is made. The mesophase pitch is preferably made with a softening point of 235 to 267 degrees Celsius and includes more than 73.1% of toluene with anisotropy of 85% to 90.1%.

For making ultra-fine carbon fibers, techniques have been devised such as blending various compounds in the form of fine particles in carbon fibers, or mixing various sorts of resin with polyacrylic polymers, or ionizing gaseous atoms or molecules, accelerating the ions in an electric field and injecting the accelerated ions into the superficial portions of the carbon fibers for improving the mechanical properties, or using vapor phase growth of nanometer-scale carbon fibers, or by dissolving phenolic resin and polyethylene in solvent to form the fine particles, or using wet spinning/dry spinning and carbonization for making ultra-fine carbon fibers after long-hours mixture of the phenolic resin and polyolefin. However, the foregoing techniques involve the disadvantage of low throughput, complicated procedures, solvent used, long production time consuming, high production cost, etc.

The present invention takes advantages and features of various methods, providing a core-sheath fiber with a core made of polyolefin polymer and carbon-containing polymers, and a sheath made of polyolefin polymer by core-sheath melt-spinning. There will be no more solvent, no more long mixing procedure is required. The core-sheath fiber is stabilized under control of temperature. The stabilized core-sheath fiber is carbonized in nitrogen at 600 to 1500 degrees Celsius and converted into ultra-fine carbon fibers with a diameter of 20 to 800 nm.

The polymer used as the carbon-containing precursor may be thermoplastic phenol-formaldehyde, thermoplastic polyacrylonitrile, thermoplastic polyvinyl alcohol, polyvinyl chloride, mesophase pitch or any combination of these polymers. By core-sheath melt-spinning, these carbon-containing polymers are made into the core-sheath fibers. Referring to FIG. 5, a core-sheath fiber includes a core 23 and a sheath for wrapping the core 23. The sheath includes fibrils 21 made from carbon-containing polymer and a matrix 22 made of polyolefin. The core 23 is made of a polyolefin polymer that can easily be melt-spinning and cannot easily be broken. The strength of the core-sheath fibers is high and undesired breach is avoided. The core-sheath fiber is stabilized under acid and aldehyde solution. The stabilized core-sheath fiber is carbonized in nitrogen at 600 to 1500 degrees Celsius, and turned into ultra-fine carbon fibers with a diameter of 20 to 800 nm. Then, the ultra-fine carbon fibers may be activated in carbon dioxide, steam, air or any combination thereof at 600 to 1500 degrees Celsius, and made into activated ultra-fine carbon fibers.

The weight ratio of the sheath to the core 23 ranges from 20:80 to 80:20. The weight ratio of the fibrils 21 to the matrix 22 ranges from 1:5 to 3:2. According to a first embodiment of the present invention, phenol-formaldehyde resin is provided in the form of chips by Dynea Company Limited. The molecular weight of the phenol-formaldehyde resin is about 3000. Polyethylene resin is provided in the form of chips, Lotrene, by Qatar Petrochemical Company Limited. Polypropylene resin is provided in chips, Pro-fax PT231. A core-sheath nozzle is operated at 205 degrees Celsius. The melt-spinning speed is conducted at 400 m/min. The sheath takes about 50% (the phenol-formaldehyde takes 20%; the polyethylene takes 30%) of the core-sheath fiber while the core 23, polypropylene, takes the remaining 50%. The core-sheath fiber is made into a stabilized (or “cross-linked”) core-sheath fiber in 18% aldehyde solution and 12% hydrochloride solution at 95 degrees Celsius. The stabilized core-sheath fiber is neutralized in ammonia water, washed and dried. The core-sheath fiber is carbonized in nitrogen at 800 degrees Celsius for about 1 hour, thus making ultra-fine carbon fibers with a diameter of 100 to 600 nanometers as shown in FIG. 6. The ultra-fine carbon fibers may be processed with steam at 1000 degrees Celsius, thus making activated ultra-fine carbon fibers with micropores.

According to a second embodiment of the present invention, mesophase pitch is provided in the form of particles, AR, by Mitsubish Gas Chemical Company. Polypropylene resin is provided in particles, Pro-fax PT231. A core-sheath melt-spinning process is used. A core-sheath nozzle is operated at 310 degrees Celsius. The melt-spinning speed is conducted at 500 m/min. The sheath takes about 60% (the mesophase pitch takes 25%; the polypropylene takes 35%) of the core-sheath fiber while the core 23, polypropylene, takes the remaining 40%. The core-sheath fiber is stabilized by extending at 60 to 200 degrees Celsius. The stabilized core-sheath fiber is carbonized in nitrogen at 1000 degrees Celsius, thus making ultra-fine carbon fibers with a diameter of 20 to 400 nanometers. The diameter may be 130 nanometers as shown in FIG. 7. The ultra-fine carbon fibers may be graphitized in argon at 2500 degrees Celsius so that ultra-fine graphite fibers are made.

As discussed above, ultra-fine carbon fibers and activated ultra-fine carbon fibers can be made in the simple and inexpensive method according to the present invention. The resultant ultra-fine carbon fibers and activated ultra-fine carbon fibers are light-weighted and thermally and electrically conductive, include a fineness of 20 to 800 nanometers and can be used in reinforced composite patch, containers for hydrogen, lithium cell electrodes, ultra-high capacitors and filters.

The present invention has been described via the detailed illustration of the embodiments. Those skilled in the art can derive variations from the embodiments without departing from the scope of the present invention. Therefore, the embodiments shall not limit the scope of the present invention defined in the claims. 

1. A method for making ultra-fine carbon fibers comprising: providing a core-sheath fiber with a core made of a polyolefin polymer, and a sheath made of a polyolefin polymer and a carbon-containing polymer, with the carbon-containing polymer being at least one selected from the group consisting of phenol-formaldehyde, thermoplastic polyacrylonitrile, thermoplastic polyvinyl alcohol, polyvinyl chloride and mesophase pitch; stabilizing the core-sheath fiber into carbon-containing precursor fiber; and carbonizing the carbon-containing precursor fiber at 600 to 1500 degrees Celsius to get ultra-fine carbon fibers with a diameter of 20 to 800 nm.
 2. The method according to claim 1, wherein the polyolefin polymer is at least one selected from the group consisting of polystyrene, polyethylene, polypropylene, polymethylpentene and an olefin-containing co-polymer.
 3. The method according to claim 1, wherein the weight ratio of the core to the sheath is in a range between 20:80 and 80:20.
 4. The method according to claim 1, wherein the weight ratio of the carbon-containing polymer of the sheath to the polyolefin polymer of the sheath is in a range between 1:5 and 3:2.
 5. The method according to claim 1, further comprising graphitizing the ultra-fine carbon fibers in Argon at 1500 to 3000 degrees Celsius.
 6. The method according to claim 1, further comprising activating the ultra-fine carbon fibers in at least one selected from the group consisting of carbon dioxide, steam and air to get activated ultra-fine carbon fibers with a diameter of 20 to 800 nm. 